The invention is directed to improvements in a fuel injection system for internal combustion engines.
In a fuel injection system of this kind, which has been disclosed by DE 43 37 048 C2, a high-pressure delivery pump supplies a high-pressure reservoir with fuel from a storage tank. Pressure lines lead from the (common rail) high-pressure reservoir to the individual injection nozzles of the internal combustion engine. The nitrogen oxide and soot emissions of the engine are reduced by means of an injection of fuel (diesel fuel) and a supplemental fluid, which can be carried out in succession. This type of injection effects a reduction in fuel consumption. As a rule, water is used as the supplemental fluid.
In a low load operation of the engine or when the engine is cold, however, the water injection portion must be dynamically and rapidly reduced or shut off. Otherwise the HC emissions increase.
In a common rail system, the high injection pressure continuously prevails in the injection nozzle. As a result, in the known fuel injection system, the provision is made that the pressure inside the pressure chamber is temporarily reduced in order to facilitate the pre-storing of supplemental fluid in the pressure chamber. This is carried out by means of a 3/2-way valve so that fuel can be displaced from the pressure chamber by means of the incoming supplemental fluid. In the injection pause, the 3/2-way valve opens the passage to a discharge line for the pressure reduction and the return of the fuel from the injection nozzle to a reservoir. At the same time, the inlet from the common rail pressure reservoir is closed off. Parallel to this valve control, an opening for the inlet of the supplemental fluid (water) is unblocked by means of another check valve. The supplemental fluid is supplied to the injection nozzle in the region of the nozzle tip. The check valve is held in an open position until the desired quantity of supplemental fluid has been pre-stored in the pressure chamber.
If the check valve is closed again, the 3/2-way valve is switched over again into the injection position for fuel. Then fuel can once again arrive in the pressure chamber at high pressure. The connection to the common rail pressure reservoir is thereby continuously reestablished. The storage of fuel and supplemental fluid in the pressure chamber occurs in layers. When supplemental fluid is pre-stored in the pressure chamber, a residue of fuel nevertheless remains beneath the inlet opening for the supplemental fluid. After being let in, the supplemental fluid forms a layer over the fuel. Then, fuel is once again supplied to the pressure chamber, which settles down over the supplemental fluid layer. Consequently, a small quantity of pure fuel is injected first at the beginning of the injection. For this reason, a short ignition delay occurs. Then all of the stored supplemental fluid is injected. The metering of the supplemental fluid through the control of the valve device must be carried out so that no residual supplemental fluid remains in the pressure chamber. At loads from zero to approx. 20% (max. 40%) full load, the storage of supplemental fluid is switched off and pure fuel is injected. At loads below approx. 20% and in engines that have not yet reached normal operation temperature, no supplemental fluid is pre-stored.
The admixture of fuel and supplemental fluid in the pressure chamber requires a precise and rapid control of the valve device. For example, depending on the operating state of the engine, the quantity of fuel or supplemental fluid must be able to be varied from work cycle to work cycle.
Two reversing valves that are separate from each other are provided in the known fuel injection system, for the filling of fuel and supplemental fluid into the pressure chamber. The reversing valve for the fuel storage is embodied as a 3/2-way valve, while in addition, a separate check valve is required for storing supplemental fluid in the pressure chamber.
The two valves must be coordinated with each other and controlled interdependently for an alternating filling of the pressure chamber. This requires additional control devices. The tuning of the valve control by means of the development of costly electronics therefore makes the fuel injection system more expensive and has a disadvantageous effect on the effort and costs associated with maintenance.
The use of the complexly designed fuel injection system, which is comprised of a number of individual components, increases the susceptibility to malfunction.